Facilitating automatic capture of high quality images with little or no user intervention is the primary objective in a camera system design for a consumer level portable electronic device, e.g., cell-phone, tablet, etc. Typically, consumers consider image sharpness to be a critical factor of final image quality. The sharpness of the final captured image is determined in significant part by the ability of the camera's auto-focus (AF) system to adjust the distance between the lens and image sensor in such a way as to render an object of interest at a certain distance from the camera onto the image sensor with an imperceptible amount of blur.
Due to its ease of hardware integration, the passive AF approach has become the de facto standard in realizing the AF feature for compact consumer portable electronic camera systems. In passive AF, a measure of image sharpness is extracted from a portion of the captured image. This measure is then used to adjust the imaging distance via a search procedure on the camera's processor so that the extracted measure obtains a peak value.
Accordingly, focus region, sharpness measure and sharpness search procedure are the three major components to a passive AF method. The goal for the sharpness measure procedure is to determine the in-focus position quickly without overshooting or oscillating around the peak as consumers desire a smooth AF experience with minimum overshoot and no rapid oscillation between extreme levels of focus and defocus. The efficiency of the search depends on the number of times the distance between the lens and the image sensor is adjusted to bring the image into focus.
In general, the camera module of a portable electronic device with photographic capabilities is configured with a Voice-Coil Motor (VCM) which is the solution with the lowest cost as well as the minimum volume for the auto-focusing function. Furthermore, the mechanical structure of the VCM is durable and vibration resistible. The main components of a VCM are a magnet and a coil, and a relative motion between the image sensor and the lens is generated by the repellence of the magnetic fields produced by the electric current going through the coil. This repellence of the magnetic fields between the magnet and the coil determines the distance between the lens and the image sensor.
Conventional portable electronic device camera modules employing the VCM solution typically run into AF problems because the components of the VCM assembly are sensitive and, as a result, gravitational force applied to the moving component may restrict or alter its movement and can influence the range of allowable movement of the lens with respect to the image sensor depending on the orientation of the device. In other words, the presence of gravity acting on the moveable component in the lens assembly acts to reduce the range of motion of a moveable component. Consequently, depending on the orientation of the device, the AF procedure can either take a longer time to find the peak value or, in the worst case, may not be able to find the peak value at all.